This invention relates to novel catalysts for the catalytic reduction of nitrogen oxides in waste gases which may additionally contain sulfur oxides. It relates also to methods of preparing the novel catalysts.
Previously known procedures for reducing waste oxides of nitrogen to nitrogen have been categorized as either wet or dry. The dry process may be catalytic oxidation, decompositions or reduction, or it may be the adsorption process. Catalytic reduction may be selective or non-selective, depending upon whether the reducing agent concurrently reacts with oxygen and the nitrogen oxides. The catalysts of this invention are useful in selective reduction utilizing ammonia as the reducing agent.
It is known that metals of the platinum group, or a metal or metal oxide selected from Ib, Vb, VIb, VIIb, and VIII group of the periodic table supported on inorganic porous carriers such as alumina, silica-alumina, silica, diatomaceous earth and the like were employed as selective reducing catalysts in the prior nitrogen oxide (NOx) removal processes using NH.sub.3 as the reducing agent. However, sulfur oxides (SOx) which often coexist with nitrogen oxides in waste gas are known to poison such catalysts so that their useful life is reduced. This problem becomes a factor at sulfur oxide concentrations as low as 1 to 10 ppm.
As ordinary flue gas normally contains 10 to 3,000 ppm sulfur oxides, the art has long sought to develop a catalyst which is resistant to poisoning by such oxides in order to carry out the nitrogen oxide removal process by catalytic reduction on an industrial scale.
Since oxygen is also usually present in waste gases, it is important that the catalyst selectively catalyze the reduction of nitrogen oxides rather than the reduction of oxygen. The reduction of oxygen is an exothermic reaction. If this reaction takes place to an appreciable extent, it becomes necessary to utilize special cooling devices and techniques which markedly increase the cost of the operation. Accordingly, another desideratum of catalysts for the selective reduction of nitrogen oxides in flue gases using ammonia as the reducing agent is that they efficiently catalyze the reaction of ammonia with nitrogen oxides, but not enhance the reaction of ammonia with oxygen.